As the energy requirements of society continue to increase and as the ability of oil to meet these requirements becomes more questionable, there has been increased incentive to develop alternate sources to supply major portions of the world's energy requirements. One such alternate source is coal which is found in abundance in many areas of the world. However, coal has certain problems associated with its use which contributed to its original decline as an energy source and to its relatively unenthusiastic acceptance as a current alternate energy source. Among these problems are the cost of transporting coal over long distances and the relatively high levels of pollutants generated when coal is burned. Both of these drawbacks are ameliorated when coal is converted to a liquid or gaseous hydrocarbon fuel. Therefore, there has been an extensive effort to expand known and to develop and commercialize new coal gasification and liquefaction techniques.
Many coal gasification and liquefaction processes require oxygen to partially oxidize the coal and thereby supply energy for the conversion process. Oxygen is preferred over air to avoid introducing large amounts of nitrogen into the conversion process and to reduce the amount of oxidant which must be compressed. Generally, the oxygen required for these coal conversion techniques is in the purity range of from about 90 to 99 percent, and more preferably, from 95 to 98 percent. Oxygen in this general purity range is often referred to as low purity oxygen. Low purity oxygen is adequate because the oxygen is used for combustion with coal which itself contains impurities.
The production of oxygen by processes employing the fractionation of air has long been known. However, most of these processes are designed to produce high purity oxygen, i.e., oxygen of at least 99.5 percent purity. As indicated above, oxygen of such high purity is not required for many coal conversion techniques. Therefore, methods which produce oxygen of low purity are becoming increasingly important.
Most methods of producing high purity oxygen by air separation employ a double column arrangement wherein a high pressure column and a low pressure column are in heat exchange relation. The high pressures required to produce high purity oxygen are not required for the production of low purity oxygen and thus many of the known low purity oxygen production methods employ just the low pressure column with an associated or side column at a medium pressure.
In a typical high purity oxygen production process employing a low pressure and high pressure column, the vapor from the top of the high pressure column is used to reboil the bottoms of the low pressure column in order to supply vapor reflux to the low pressure column. Although there do exist low purity oxygen production processes which employ the vapor from the top of the medium pressure column to effect this reboil, it is generally preferable to employ compressed feed air to reboil the low pressure column bottoms because of the relatively higher operating pressures of the medium pressure column required by the former methods.
Methods which employ feed air to reboil the low pressure column bottoms are generally termed "air boiling" methods and several such methods for producing low purity oxygen are known in the art.
One such method is described in U.S. Pat. No. 2,850,880--Jakob wherein air is supplied at a single head pressure to both the low pressure column main condenser and the medium pressure ccolumn as a feed. Another such method is described in U.S. Pat. No. 4,208,199--Nakazato et al. wherein air is supplied at a single head pressure to both the low pressure column main condenser and to the medium pressure column as feed. The liquid air which is condensed to effect the reboil is fed to the medium pressure column as additional feed.
Both of these prior art processes suffer from the requirement of supplying air to the medium pressure column at the same pressure required by the main condenser. The pressure of the air to the main condenser must be relatively high in order to achieve good reboil of the low pressure bottoms. Thus, these single head pressure methods are disadvantageous from the standpoint of energy efficiency.
Another prior art process is described in U.S. Pat. No. 2,209,748--Schlitt. This process uses air feed to the medium pressure column at a lower pressure than that employed in the main condenser. However, this method requires a two-step air condensation and as such requires a very high pressure in the main condenser; in effect, it approximates a high pressure column separation and is therefore also inefficient from an energy use standpoint.
A method to produce low purity oxygen by rectification of air in a medium pressure column and a low pressure column wherein feed air is employed to reboil the low pressure column bottoms which is more efficient than heretofore known such methods would be highly desirable.
It is therefore an object of this invention to provide an energy efficient process to produce low purity oxygen by fractionation of air employing a medium pressure column and a low pressure column.
It is another object of this invention to provide an energy efficient process to produce low purity oxygen by fractionation of air employing a medium pressure column and a low pressure column wherein feed air is employed to reboil the bottoms of the low pressure column.